1. Field of the Invention
The present invention relates to detection of automobile exhausts, and, more particularly, to the detection of carbon monoxide gas in the presence of other gases.
2. Description of Related Art
A sensor for CO gas has immediate application to automobiles for monitoring catalytic converter efficiency. Sensors placed before and after the catalytic converter would be able to quantitatively monitor the oxidation efficiency of environmentally harmful CO by the converter. Another automobile application includes real-time control of engine processes via a feedback loop of exhaust composition from a strategically located sensor.
A secondary application is for CO monitoring in a general sense. This could include the quantitative determination of CO present in fuel cells, or a simple detection of CO in laboratories, mines, and industrial smoke stacks.
Semiconductive metal oxides have been used as CO and hydrocarbon sensors. Two major problems are encountered, however. The first is a non-linearity in the signal response when the mixture of oxidizable gas and oxygen reaches a stoichiometric ratio for combustion. For the case of CO, this occurs when the CO/O.sub.2 ratio equals 2:1 in the following reaction: EQU 2CO+O.sub.2 =2 CO.sub.2. (1)
When this ratio is attained, the signal can change by over an order of magnitude, and the calibration of sensor is therefore lost.
The second problem encountered with metal oxide sensors is a lack of detection specificity with respect to an individual gas in a gas mixture. This can be a particular problem in the case of automobile exhaust, where signal interference can arise from the presence of NO(g), H.sub.2 O vapor, and other hydrocarbon gases resulting from fuel combustion.
Thus, there is a need for a CO sensor that is substantially free of the problems of the prior art sensors.